A typical gas turbine engine comprises a compressor, at least one combustor, and a turbine, with the compressor and turbine coupled together through an axial shaft. In operation, air passes through the compressor, where the pressure of the air increases and then passes to a combustion section, where fuel is mixed with the compressed air in one or more combustion chambers and ultimately ignited. The hot combustion gases then pass into the turbine and drive the turbine. As the turbine rotates, the compressor turns, since they are coupled together along a common shaft. The turning of the shaft also drives a generator for electrical applications. The engine must operate within the confines of the environmental regulations for the area in which the engine is located. As a result, more advanced combustion systems have been developed to more efficiently mix fuel and air so as to provide more complete combustion, which results in lower emissions.
As the demand for more powerful and efficient turbine engines continues to increase, it is necessary to improve the efficiency at each stage of the turbine, so as to get the most work possible out of the turbine. To achieve this efficiency improvement, it is necessary to remove any design defects that limit the turbine from achieving its maximum performance. Turbine blades have been known to be limited in power output by a variety of conditions including, but not limited to creep, flutter, and erosion.
Flutter is a dangerous condition caused by the interaction of an airfoil's structural modes of vibration with the aerodynamic pressure distribution on the blade. As the airfoil portion of the turbine blade vibrates, its pressure magnitudes and distributions fluctuate due to the changing flow path geometry. This can result in energy being either added to the flow (a condition know as positive aero-damping) or energy being extracted from the flow (negative aero-damping). If the energy being extracted from the flow is greater than can be dissipated through mechanical damping, the amplitude of the displacements will increase. The cycle repeats itself and is compounded until either the energy input and energy dissipated balance each other, or failure occurs. In order to avoid excessive flutter, which can cause component failure, limitations may be placed upon the operating condition of the turbine. Furthermore, excessive flutter outside of acceptable limits can cause the turbine blade to fail over time.